Hydrogen sulfide and other odorous sulfide compounds result from the usage of sulfates and other compounds containing sulfur in anaerobic conditions by sulfate reducing bacteria (SRB) that are ubiquitous in wastewater collection and treatment systems. The problem is exacerbated by higher temperatures encountered in the summer and in more tropical climates. Hydrogen sulfide has an obnoxious characteristic rotten egg odor and is toxic even in low concentrations. It is also highly corrosive to metal and concrete components of wastewater collection and treatment systems. Because of its offensive odor and toxicity, regulatory restrictions on hydrogen sulfide emissions from wastewater collection and treatment systems and other emission sources are becoming more stringent.
The addition of oxidizing substances such as chlorine, hydrogen peroxide, ozone, oxygen, and potassium permanganate to wastewater collection and treatment systems to control hydrogen sulfide odors is common. These are typically metered into wastewater at points in the collection system that are upstream of sites where anaerobic conditions prevail, and they reduce the hydrogen sulfide concentrations by oxidizing the hydrogen sulfide, and by providing aerobic conditions to inactivate the SRB. These substances are very strong oxidants and effectively oxidize the hydrogen sulfide that is present, but are rapidly depleted through the oxidation of other components of the wastewater, and do not have a sustained effect. Any oxygen that may be released is used by the natural bacteria in the wastewater and is rapidly depleted. When the oxidant is depleted the system again becomes anaerobic, reactivating the SRB to again generate hydrogen sulfide which must be treated again by the oxidizing compounds.
The addition of nitrate ions to wastewater to control hydrogen sulfide odor is also common. Nitrate not only chemically oxidizes hydrogen sulfide but also provides a terminal electron acceptor for certain facultatively anaerobic bacteria present in wastewater to effectively compete with the SRB for the available energy and carbon sources, thus inhibiting the production of hydrogen sulfide. Nitrate also acts as an electron acceptor for certain other bacteria that oxidize hydrogen sulfide to sulfate or other benign oxidized forms of sulfur. For the addition of nitrate to be effective in controlling concentrations of hydrogen sulfide the nitrate must be present at adequate levels continuously in the collection and treatment system. This requires providing numerous addition points throughout the system to assure adequate levels.
It is also known to add certain selected bacteria to wastewater systems to effectively compete with the SRB for the available energy and carbon sources and thus effectively inhibit the production of hydrogen sulfide. To be effective these bacteria require a source of energy such as nitrate or nitrite, which is normally present in wastewater, but often becomes depleted due to bacterial activity, or events that alter the flow in the system.
In addition to controlling odor by decreasing the hydrogen sulfide concentration it is also desirable to decrease the biological oxygen demand (BOD), chemical oxygen demand (COD), total suspended solids (TSS) fats, oils, and greases of the wastewater in the collection system, before it reaches the treatment plant. This activity results in reduced foaming, scum formation and sludge volumes in the treatment plant.
Water in oil emulsions containing high concentrations of nitrate salts are well known, and are extensively used in explosives containing ammonium nitrate. Similar emulsions containing water soluble fertilizer materials including nitrates have been proposed as slow release fertilizers.
The object of this invention is to provide a method for the reduction of hydrogen sulfide concentrations in wastewater collection and treatment systems.
Another object is to provide for the addition of bacteria that rapidly and effectively metabolize the available carbon sources to prevent the activity of the SRB.
Another object is to provide a nitrate source to enhance the growth and metabolic activity of the added bacteria when oxygen is absent.
Another object is to provide a nitrate source to the naturally occurring bacteria in the wastewater that require nitrate during their metabolism in oxidizing hydrogen sulfide to sulfate, elemental sulfur and other forms of sulfur.
A further object is to provide for the slow release of the nitrate to assure its presence throughout the system with minimum dosages and addition points.